This invention relates to wrought and recrystallized lead and lead alloys, with increased resistance to creep and intergranular cracking and corrosion. This invention is more particularly concerned with positive lead and lead alloy electrodes used in lead-acid batteries which, via recrystallization treatment to generate new grain boundaries in the microstructure, have improved resistance to corrosion and growth, so as to provide enhanced battery reliability, extended service life and greater energy density.
Intergranular degradation (i.e., creep deformation, cracking and corrosion) of lead-based positive electrode materials are the principal cause of premature failure of lead-acid batteries. Intergranular corrosion occurs from the change in volume associated as PbSO4 is deposited in grain boundaries intersecting the surface (during discharge) and is transformed to PbO2 during the charging cycle. As intergranular corrosion occurs, the lead-based electrodes break down and the performance of the battery deteriorates.
Creep deformation, which arises primarily from grain boundary sliding processes, results in dimensional expansion of the positive electrode. a so-called xe2x80x9cgrowthxe2x80x9d which causes: (1) loss of contact between the electrode surface and the PbO2 paste and/or (2) contact/shorting between adjacent electrodes leading to losses in capacity. The growth of the positive electrode also contributes to intergranular xe2x80x9ccrackingxe2x80x9d.
Growth of the positive electrode in lead-acid batteries has become the predominant concern with automotive xe2x80x98starter, lights and ignitionxe2x80x99 batteries as under-the-hood temperatures rise in modem automobiles. As a result of these intergranular degradation processes, and in order to maintain sufficient operating- and cycle-life performance, considerable thickness allowances are required on the minimum dimension of the positive electrodes, which commensurately increase the overall size and weight of the batteries.
Early improvements in positive lead electrodes were obtained by alloying the lead with: Sb, Sn, As, Ca and other elements. These efforts were made to strengthen the alloys by precipitation or age hardening, such as are disclosed in the U.S. Pat. Nos. 4,753,688 to Myers, 1,675,644 to Dean and 3,888,703 to Tilman, all of which are directed to antimony-bearing lead alloys. Precipitation and age hardening techniques require the presence of an alloying element which is not soluble in lead at ambient or operating temperature which forms a second phase in the metal. Hardening is typically achieved by straining and then heat treating the lead alloy above the solvus temperature, to solutionize the second phase, and then quenching the metal to form a supersaturated solution of the alloyed element in the lead. Over time, the alloyed element precipitates out of solution to form a second phase, preferably in the form of small precipitates, in the metal. These second phase precipitates impede dislocation motion in the metal, inhibit grain boundary sliding, and consequently strengthen and harden the material. Quenching following the heat treatment is necessary to keep the precipitate size small and effective in terms of strengthening and growth resistance. The deformation prior to heat treatment, typically achieved through cold or hot working, forms dislocations in the crystallographic structure of the metal which act as the nucleation sites for the precipitation of the second phase, and result in a more uniform precipitate distribution.
It should be noted that as a result of the relatively low melting temperature of lead and lead alloys, precipitation hardening typically occurs at room temperature. The techniques taught in the prior art, as exemplified in the above listed patents, are primarily directed to reduction of the time required to achieve optimum strength, from a few days at room temperature to a few minutes at elevated furnace temperatures.
There has also been a general recognition by the lead-acid battery industry, that wrought lead-alloys which are cold worked following casting of the molten alloys, yield enhanced growth resistance relative to lead and lead alloys which are simply cast to final shape. This performance improvement has been attributed to xe2x80x98microstructuralxe2x80x99 refinement, and examples are outlined in U.S. Pat. Nos. 5,611,128 and 5,604,058 to Wirtz, which describe processes to cold roll near net shape battery electrodes from cast grid blanks. The benefits obtained from such wrought lead alloys may also be attributable to precipitation processes whereby uniform precipitate distribution is obtained by longer term aging at ambient temperature. In this regard, it should be noted that performance improvements using xe2x80x98wroughtxe2x80x99 electrodes have been observed only with lead alloys containing alloy constituents such as Ca, Sn, Sb, Ba etc., which are insoluble at ambient temperature, and form precipitates on aging. Moreover, both precipitation-processed and wrought electrodes have not been shown to display any significant improvements with regard to intergranular corrosion.
Although xe2x80x98precipitation hardeningxe2x80x99 processes, involving the proper choice of alloying constituents, and prior cold working to enhance the uniformity of precipitate distribution from aging at ambient or elevated temperature, undoubtedly have a beneficial impact on minimizing grid growth from grain boundary sliding (i.e., grain boundary xe2x80x9cpinning by precipitatesxe2x80x9d). We have found that it is preferable to alter the structure of grain boundaries in the material directly, not only to impede grain boundary sliding, but also to minimize intergranular corrosion and cracking susceptibility. Unlike precipitation-based processes, such a new approach, according to the present invention, is also applicable to pure lead and lead alloys not containing precipitate-formers. This opens the way to the advantageous use of less expensive alloys.
Various studies have shown that certain special grain boundaries, described on the basis of xe2x80x9cCoincident Site Latticexe2x80x9d model of interface structure (Kronberg, and Wilson. Trans. Met. Soc. AIME, 185, 501 (1949), as lying within Dq of xcexa3, where xcexa3xe2x89xa629 and Dqxe2x89xa615xc2x0xcexa3xe2x88x92xc2xd (Brandon, Acta Metall., 14, 1479 (1966)) are highly resistant to intergranular degradation processes such as corrosion, cracking, and grain boundary sliding; the latter being a principal contributor to creep deformation. However, these studies provide no instruction as to how to achieve a high concentration of special grain boundaries, and as noted, it is only recently that techniques such as Orientation Imaging Microscopy have become available, to enable grain boundaries to be studied. Moreover, the only means of creating new grain boundaries during solid state processing is to effect recrystallization of a material by cold working followed by suitable heat treatment; such a novel approach to the processing of lead acid battery positive electrodes therefore forms the basis of the present invention.
In previously issued U.S. Patents by one of the present inventors, U.S. Pat. Nos. 5,702,543, and 5,817,193, a thermomechanical process is disclosed for increasing the population of such special grain boundaries in commercial austenitic Fe and Ni-based stainless alloys from approximately 20-30% to levels in excess of 60%; such an increase resulting in significantly improved resistance to intergranular degradation processes such as intergranular corrosion and stress corrosion cracking. However, the process described and claimed in that patent is directed exclusively to certain austenitic stainless steels and nickel-based alloys, and not with any other metals. The intended application of such alloys and the environment they encounter in use is quite different from the harsh, acidic environment of lead-acid batteries.
In accordance with the present invention there is provided a lead or lead alloy, which has been processed to substantially increase the percentage of special grain boundaries, thereby to increase at least one of the resistance of the lead or lead alloy to creep and resistance to intergranular corrosion and intergranular cracking, wherein the lead or lead alloy has been subjected to at least one processing cycle comprising: cold working or straining the lead alloy by a substantial amount, preferably in excess of 10%; and subsequently annealing the lead alloy for a time and temperature sufficient to effect recrystallization to substantially increase the concentration of special grain boundaries.
In this specification, including the claims: a reference to lead means either pure lead or a lead alloy; a reference to cold working means any forming operation such as rolling extruding etc. conducted at ambient or room temperature, a reference to straining means application of a either a compressive or tensile plastic strain (e.g., expansion); a reference to lead alloy denotes an alloy that includes one or more specific alloying elements.
Preferably, the steps of cold working or straining the lead alloy and annealing to recrystallize the lead alloy are repeated a plurality of times. Excessive strain between recrystallization steps can have a negative effect on the present process. However, for lead alloys, unlike other metals, the inventors have surprisingly found that, at least for some alloys, a desired concentration of special grain boundaries can be obtained with a single step of cold working or straining and annealing.
The lead alloys may be comprised of at least one alloying element selected from the group comprising, tin, barium, calcium, selenium, bismuth, silver, iron, arsenic, copper and zinc, but the alloy can also include two or more alloying elements. The alloying element(s) need not be soluble in lead. In the case of substantial alloys, the lead alloy is preferably reduced in thickness or strained by approximately 10%-80% in each cold working step, and the lead alloy is then recrystallized, in the annealing step, at a temperature and time sufficient to allow recrystallization to occur, generally in the range of approximately 150xc2x0 to 280xc2x0 C. for 10 seconds to 10 minutes and subsequently air-cooled to ambient temperature with no quenching required. It is to be appreciated that the exact deformation and annealing temperature and time required for recrystallization and the formation of special grain boundaries will vary depending on the alloying additions and the percentages added.
Preferably, in the processed lead and lead alloys, the percentage of special grain boundaries is at least 50% of the total grain boundaries. For pure lead and many lead alloys, it has been found that the percentage of special grain boundaries in the processed lead can be increased to at least 60% of the total grain boundaries.
In accordance with another aspect of the present invention, the lead or lead alloy is subsequently processed into components for lead-acid batteries, for example electrodes. It is preferred for the lead or lead alloy to be subject, first, to processing according to the present invention, and that this processing be applied uniformly to all the lead. The degree of uniformity may depend on the method of cold working or straining the lead alloy, e.g. stamping, extrusion, rolling, expanding, forging etc., and component geometry.